In existing designs, cutting plates have a generally rectangular body with a profiled tip for cutting a workpiece, for example, masonry. The body is welded, brazed or fixed by other mechanical means to a drilling head located at one end of a shaft of a drill bit having a longitudinal axis. The profiled tip of the cutting plate protrudes axially from the drilling head. Located at the other end of the shaft is a shank to releasably connect to a tool holder or a drill or the like, for example, a rotary drill, a rotary percussion drill or a rotary hammer. The shaft also has a conveying flute extending axially rearward from the drilling head toward the shank end of the drill bit. The flute may be a single-start or multi-start, straight or helical flute or a combination thereof. The primary function of the flute is to convey debris broken by the cutting plate out of the hole drilled in a workpiece. In use, the shank of the drill bit is connected to and rotated by the tool holder while a percussive force is simultaneously applied to the shank in an axial direction. The rotary movement and the percussive force are both transmitted to the cutting plate which is in contact with a masonry workpiece. The percussive force transmitted to the rotating cutting plate helps it to drill a circular hole in the workpiece.
Commonly, masonry drill bits comprise a single cutting plate. The cutting plate's profiled tip has a pair of cutting edges extending radially outward from the drill bit's axis to the periphery of the drill bit's working diameter. Each cutting edge is defined by the transition between a leading rake face and a trailing relief face. Both the rake and the relief faces are flat and slope axially rearward from the cutting edge to give the cutting plate's profiled tip a roof-like shape. A conveying flute starts on each side of the cutting plate. A single cutting plate is satisfactory for masonry drill bits having a diameter up to approximately 16 mm. However, the performance of a single cutting plate becomes decreasingly satisfactory for masonry drill bits having a diameter over and above 16 mm. This is because, in use, the space swept by the cutting edges between impacts of the percussive force is significant at the periphery of a working diameter over and above 16 mm.
The impacts of the percussive force cause the radially orientated cutting edges to cut successive radial notches in the workpiece as the cutting plate rotates. This is part of, the normal process of drilling a hole in a masonry workpiece, however, substantial vibration occurs if these radial notches become too rough. The vibration propagates from the cutting plate along the shank of the drill bit and eventually to the user, via the power tool. This can be very uncomfortable for the user. Also, the rotational speed and the amount of material removed per revolution by the cutting edges is highest at the periphery of the working diameter. Also, frictional forces acting in that region can become intolerably high for such drill bits. Excessive frictional forces cause overheating, premature wear and damage to the cutting edges at the periphery of the working diameter. This destabilises the drill bit and impairs its ability to drill a clean hole in the workpiece, as well as shortening the lifespan of the drill bit. These problems are present in all masonry drill bits comprising a single cutting plate, however, they are more pronounced in masonry drill bits comprising a single cutting plate having a larger diameter.
These problems are addressed by providing masonry drill bits of larger diameter with at least one auxiliary cutting plate fixed to the drilling head at the periphery of the working diameter, in addition to the main cutting plate. Like the main cutting plate, each auxiliary cutting plate generally has a profiled tip with a cutting edge. The cutting edge is defined by the transition between a leading rake face and a trailing relief face. Both the rake and the relief faces are flat and slope axially rearward from the cutting edge to give the auxiliary cutting plate's profiled tip a roof-like shape. Often an additional conveying flute is required to start from each additional auxiliary cutting plate. Auxiliary cutting plates provide additional cutting edges which have the effect of smoothing the radial notches cut in the workpiece at the periphery of the working diameter to reduce the vibration propagating along the drill bit. The cutting edges of the auxiliary cutting plates also spread the frictional load more evenly around the periphery of the working diameter. This reduces the frictional load per unit length acting on the cutting edges of the main cutting plate to a tolerable level. This overcomes the problems identified above which results in drilling a cleaner hole in a workpiece, even one drilled in hard masonry material. The cutting edges of the auxiliary cutting plates also increase the speed at which the drill bit cuts a hole in a workpiece.
An example of a masonry drill bit with a main cutting plate and a single auxiliary cutting plate is disclosed by patent publication no. EP347602B. An example of a masonry drill bit having a main cutting plate and a pair of symmetrical auxiliary cutting plates is disclosed by patent publication no. EP687617B. The auxiliary cutting plates of both these disclosures have profiled tips with the common roof-like shape generated by the pairs of flat rake and relief faces sloping axially rearward from the cutting edges.
While auxiliary cutting plates do enhance the performance of drill bits of larger diameter, they are nonetheless additional components that add cost and complexity to the design and manufacture of drill bits. Such drill bits are not inexpensive and manufactures are keen to provide end users With a drill bit with a satisfactory life span. By its very nature, a drill bit is subject to wear and there comes a time when the cutting edges are blunted to the extent that drilling speed is impaired and the drill bit becomes obsolete and needs replacement.
Cutting plates made of hard composite materials like, for example, tungsten carbide have good wear characteristics. These extend the normal lifespan of masonry drill bits beyond that expected of hardened steels. As technology advances new composite materials for cutting plates are developed which are increasingly hard. These new materials are more expensive but this is justified by even better wear characteristics which make such new materials cost effective.
Wear on the cutting edges dictates the ultimate lifespan of a masonry drill bit; however, a chipped cutting edge or complete removal of a cutting plate can result in premature failure of the drill bit. In the case of cutting plates made from hard composite material, re-profiling of a chipped cutting edge is not possible. The only option is completely removing and replacing of the cutting plate. If the cutting plate is brazed or welded to the drilling head of the drill bit such a course of action will be costly and time consuming and will cause withdrawal of the drill bit from service. A spare drill bit will be needed to continue the job at hand, but only if the user has taken the precaution of keeping spares or has the financial means to do so. Otherwise, the job at hand will cease, which is undesirable. If the cutting plate has been completely removed during operation of the drill bit, which may not be immediately noticed, continued use of the drill bit may cause collateral damage to the drilling head. Drill bits are designed to withstand wear to the cutting plate and cutting edge but not wear and/or damage to the drilling head. Accordingly, inadvertent damage to the drilling head may be of the nature to cause premature failure of the entire drill bit. For these reasons, it is of utmost importance that drill bit are designed in a manner that reduces the likelihood of damage to the cutter plate and, in particular, complete removal of the cutting plate when the drill bit is in use.
Frequently, cutting plates are seated in a slot in the drilling head and fixed by brazing. In the case of a drill bit comprising a main cutting plate and at least one auxiliary cutting plate, it is the auxiliary cutting plate that mostly risks complete removal. This is because the main cutting plate is seated in a slot in the drilling head spanning the full working diameter, whereas each auxiliary cutting plate is seated in a narrow slot in the drilling head located at the periphery of the working diameter. Accordingly, the main cutting plate is brazed to the drilling head along its broadest lateral faces which provides greater adhesion than the auxiliary cutting plate, which is only brazed to the drilling head along narrow lateral faces and one side face. Further, frictional contact with the cutting face and the sidewall of the drilled hole creates shear forces that tend to pull a cutting plate radially outward from its slot. The main cutting plate spans the working diameter and thus any such radial forces are cancelled out. Whereas, only the brazed joint between the auxiliary cutting plate and its slot acts against such radial forces. Accordingly, it is of utmost importance that the auxiliary cutter's brazed joint is defect free. Otherwise, it will be prone to failure in use resulting in removal of the auxiliary cutting plate and the likely consequential damage to the drilling head mentioned above. This is something that preoccupies the mind of all drill bit designers who strive to find solutions to the problem of removal of auxiliary cutting plates in use.